In electronic assemblies, it is known to use transmission lines to transmit an electromagnetic signal between components on a printed circuit board. Microstrip and strip line are illustrative examples of transmission lines which are used to transmit electromagnetic signals between components on a printed circuit board. Microstrip and stripline feature planar metallic conductors that are readily amenable to fabrication during the construction of a printed circuit board.
Microstrip comprises a conductive strip separated from a ground plane by a dielectric layer. Stripline comprises a central conductive strip adjoined by two dielectric layers. The two dielectric layers separate the central conductive strip from two ground planes. Microstrip and stripline technology have been improved primarily by the introduction of new dielectrics which offer lower propagational loss. However, because propagation in microstrip and stripline is limited to the transverse electric mode, microstrip and stripline may be cumbersome to tune. Moreover, the single propagational mode combined with dielectric constraints confluence to form practical operating impedance limitations for microstrip and stripline technology.
It is known to use waveguides, such as hardlines, to transmit electromagnetic signals between different circuit boards. Waveguides generally offer lower propagational loss than microstrip and stripline and greater isolation of electromagnetic radiation than microstrip and stripline do. Waveguides may also offer increased ability for controlling impedances than microstrip and stripline line do.
A waveguide typically has a hollow cross-section, which determines the propagation mode and the cut-off frequency of electromagnetic radiation transmitted by the waveguide. The complexity of manufacturing suitable elliptical, circular, and rectangular cross sections has prevented the incorporation of integral waveguides into printed circuit boards. Thus, while metallic waveguides with sundry geometrical cross-sections are commercially available as discrete transmission line components, so far microstrip and stripline technology generally reflect the limited extent of waveguide incorporation into printed circuit boards of commercially available products.
The impedance of microstrip and stripline is determined by the width of the conductive strip, the thickness of the dielectric layer or layers, and the dielectric constant of the dielectric material. If the thickness of the dielectric material is sufficiently thin, such as 50 microns or thinner, then manufacturing tolerances must be carefully controlled to compensate for potential variances in the dielectric thickness. Variances in dielectric thickness may cause line impedance mismatches. Even slight variations in the dielectric thickness of ten to fifteen microns can detrimentally affect the impedance of stripline or microstrip, in which a dielectric layer is 50 microns or thinner. Batch-to-batch variations in the dielectric material properties further contribute to impedance mismatches associated with transmission line using thin dielectric layers of 50 microns or less. While many commercially available discrete waveguides offer uniform impedances, the difficulty and expense of integrating discrete waveguides into printed circuit boards has discouraged the use of integral waveguides to facilitate impedance matching of intraboard signals.
Thus, a need exists for an integral waveguide for intraboard connections of a printed circuit board. In addition, a need exists for an integral waveguide that can provide a substantially constant impedance despite ordinary variations in dielectric thickness from manufacturing procedures.